1. Field of the Invention
The present invention relates to a DC—DC conversion circuit for use in a drive circuit of a liquid crystal display device, and the like, and particularly, to a DC—DC conversion circuit formed by use of thin film transistors using polysilicon as a material.
2. Description of the Related Art
A liquid crystal display device has a configuration in which a liquid crystal layer is held in a gap between a glass-made array substrate and a glass-made opposite substrate arranged opposite to the array substrate, the array substrate including a pixel display unit having pixels arranged on intersections of a plurality of signal lines and a plurality of scan lines.
In recent years, a manufacturing technology for forming thin film transistors (TFTs) using polysilicon as a material on the array substrate has been making a progress, and use of this technology has made it possible to form the pixel display unit and a drive circuit which drives the signal lines and the scan lines on the same array substrate. In this forming technique, the thin film transistors are used for both of transistors arranged on the respective pixels and transistors arranged on the drive circuit, and both of the transistors are formed in the same manufacturing step (for example, refer to Japanese Patent Laid-Open Publication No. 2001-343945).
When driving the liquid crystal, a plurality of drive voltages are required, and the drive circuit requires a DC—DC conversion circuit. Use of the thin film transistors for transistors forming this DC—DC conversion circuit has made it also possible to form the DC—DC conversion circuit on the array substrate.
However, the thin film transistors have had problems that there are large variations in characteristics thereof, causing variations of output voltages of the DC—DC conversion circuit.